Method of treating matte tin plate to prevent darkening



United States Patent 3,216,912 METHOD OF TREATING MATTE TIN PLATE TOPREVENT DARKENING Tom L. Shoemaker, Edgewood, Pa., assignor to UnitedStates Steel Corporation, a corporation of New Jersey No Drawing. FiledSept. 5, 1961, Ser. No. 135,787 1 Claim. (Cl. 204-37) This inventionrelates to the manufacture of tin plate and, in particular, to a methodof making matte tin plate, i.e., tin plate coated by the electrolyticprocess, which is not heated to a temperature above the melting point oftin after coating to effect melting or brightening of the surface.

Matte tin plate is used extensively and, like the surface brightenedproduct, is subject to discoloration on storage evidenced by theformation of yellow stain. This tendency can be cured by chemicaltreatment but, in the case of matte tin plate, the treatment which ismost effective gives rise to another undesirable effect, i.e., darkeningduring the baking incident to lacquering and lithographing. Suchdarkening is particularly objectionable in cases where portions of thetin surface are left un-inked to provide a light background forlithographed design.

I have invented a novel method for making matte tin plate whicheffectively eliminates the bake-darkening tendency of plate which hasbeen treated in the conventional manner to inhibit yellow stain. I havefound that mattee tin plate, if heated briefly to a temperatureapproaching but below the melting point of tin, may then be given theusual chemical treatment to resist yellow staining without developingthe tendency to brake darkening which has heretofore been the result ofsuch treatment. More particularly, after plating with tin, I heat thematte tin plate to a temperature between 420 and 445 F., for a fewseconds then quench it in water and subject it to cathodic electrolysisin a sodium-dichromate solution. The dichromate treatment provides thedesired yellowstain resistance and my pre-heating keeps the product freefrom darkening during subsequent baking incident to lacquer coating orlithographing.

A complete understanding of the invention may be obtained from thefollowing detailed description of a typical example thereof.

A continuous strip of tin plate was produced in the conventional mannerby electroplating tin on a base of low-carbon steel. After beingelectroplate, the strip was dried and then heated to a temperature of430 F. by passing it through an induction-heating coil (10,000 cyclesper second). The moving strip was held in this temperature forapproximately three seconds by supplemental resistance heating. For thispurpose sixty-cycle alternating current was supplied to the strip by twoconductor rolls spaced 34 feet apart. The temperature of the strip wasthen lowered abruptly by passing it through a water-quenching tank. Thestrip was next passed between anodes immerse din an aqueous solutioncontaining 24 grams of sodium dichromate per liter, adjusted to a pH of4.7, at a temperature of 125 F. The strip was made cathode in thesolution and an electric current of 52 coulombs per square foot of stripwas passed between the strip and the anodes. After emerging from thetreating solution, the strip was rinsed with cold water, dried withsteam, and electrostatically oiled.

Subsequent analysis of the treated strip indicated presence of 0.65milligram of chromium (calculated as metallie chromium) per square footof strip surface. Despite the presence of chromium in this quantity,however, the surface of the strip did not darken when baked atlacquercuring temperature (375 F.) for thirteen minutes; in fact, thesurface brightened slightly.

Another section of the same strip was treated in the same manner asdescribed above, except that the heating step was not included. Afterthe strip was baked, the total reflectance from its surface was 10% lessthan that from the surface of the strip processed by the method of myinvention.

The duration of the heating step prior to electrochemical treatment maybe varied from 0.1 to 10 seconds at the maximum temperature, dependingon the rate of heating and and the maximum temperature attained. Theheat may be applied by any convenient method consistent with the speedof the strip through the plating line. All electrolytic tin-platinglines include a melter for flow-brightening the tin surface when abright product is desired. In present practice, the melter is not usedduring the production of matte tin plate. The method of my invention maytherefore be practiced commercially by merely operating the melter,utilizing either resistance or induction heating or both, with a reducedelectric current to heat the strip to a temperature approaching butbelow the melting point of tin.

It will be evident from the foregoing that my invention makes possiblethe production of matte tin plate which is protected against yellowstaining but is not subject to darkening when subsequently baked.

Although I have disclosed herein the preferred embodiment of myinvention, I intended to cover as well any change or modificationtherein which may be made without departing from the spirit and scope ofthe invention.

I claim:

In a method of making matte tin plate, the steps comprisingelectroplating a tin coating onto a sheet-steel base giving it a matteapearance, then, while maintaining the temperature of said basecontinuously below the melting point of tin, heating the tin plate to atemperature of from 420 to 445 F. for from 0.1 to 10 seconds, thenimrmediately quenching the tin plate and subjecting it to electrolysisas cathode in an aqueous solution of an alkali-metal dichromatecontaining about 24 grams of said dichromate per liter, thereby leavingthe tin plate with its initial matte appearance and reducing its normaltendency to darken on subsequent baking.

References Cited by the Examiner UNITED STATES PATENTS 1,435,260 11/22Russ. 2,357,126 8/44 GlOck 204-37 2,381,778 8/45 Schoolnmaker. 2,450,50910/48 Glock 20436 2,503,217 4/50 Prust 20414l 2,606,866 8/52 Neish204-29 2,974,091 3/61 Neish 204-35 3,062,725 1 1/ 62 Frankenthal.3,087,871 4/63- Kamrn.

FOREIGN PATENTS 729,914 5/55 Great Britain.

0 JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, MURRAY TILLMAN, WINSTON A. DOUGLAS, Examiners.

